As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on batteries satisfying various needs has been carried out. Especially, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high output stability, is very high.
One of the principal problems to be solved in connection with the lithium secondary battery is to improve the safety of the battery. For example, the interior temperature and pressure of the lithium secondary battery may be greatly increased due to the abnormal operation of the battery, such as an internal short circuit, overcharge exceeding allowable current and voltage, exposure to high temperature, dropping, pressing, and nail penetration. According to circumstances, the battery may catch fire or explode.
Therefore, a method of mounting a protection circuit or a protection element and a method of using thermal choking through a separator have been proposed as a method of improving the safety of the battery. However, the use of the protection circuit acts as a great restriction in reducing the size and manufacturing costs of a battery pack. Especially when the protection circuit malfunctions, it is not possible to secure the safety of the battery when the battery is overcharged. Consequently, the use of the protection circuit is not a fundamental solution. On the other hand, the thermal chocking mechanism through the separator does not effectively operate when the generation of heat rapidly progresses.
In addition, a method of adding an organic electrolytic solution has been proposed to solve the safety-related problem of the battery. For example, U.S. Pat. No. 6,074,776 discloses an example of overcharge prevention using a monomer that can be polymerized. Furthermore, Japanese Patent Application Publication No. 2002-215909 discloses an example of 1 to 10 weight percent of a side-chain polycyclic aromatic compound and benzene compound to an organic solvent. When the organic electrolyte additive is used, however, the performance of the battery is deteriorated during the normal operation of the battery.
Also, Japanese Patent Application Publication No. 2002-164206 discloses a method of preventing overcharge, which is different from the above-mentioned method of using the electrolyte additive. According to this method, a conducting agent, such as carbon black, and a binder are previously applied to a cathode current collector, and then a cathode active material and a binder are applied to the cathode current collector. In this case, the resistance of the conducting agent layer increases by approximately 100 times, when the battery is overcharged, to interrupt electric current. Besides, there have been proposed methods of modifying the surface of an active material to improve the safety of the battery. For example, Japanese Patent Application Publication No. 1997-055210 discloses a cathode active material manufactured by coating lithium-nickel oxide with alkoxide, such as Co, Al, or Mn, and heat-treating the lithium-nickel oxide. Japanese Patent Application Publication No. 1999-016566 discloses lithium-based oxides coated with metal, such as Ti, Sn, Cu, Si, Ga, W, Zr, B, or Mo, and oxides thereof. Japanese Patent Application Publication No. 1999-185758 discloses a cathode active material manufactured by coating lithium-manganese oxide with metal oxide, using a co-precipitation method, and heat-treating the lithium-manganese oxide.
However, the above-described methods do not increase the initial temperature at which the reaction between the surface of the active material and an electrolyte occurs, i.e., the temperature at which oxygen, coupled with the metal of the cathode active material, is separated from the metal of the cathode active material (exothermic temperature), during the charge of the battery. In addition, the above-described methods do not reduce the amount of heat generated.
U.S. Pat. No. 5,705,291 discloses a method of increasing the structural stability of a cathode active material by coating the cathode active material with a compound including borate, aluminate, silicate, or a mixture thereof. However, this method does not sufficiently improve the structural stability of the cathode active material.
Furthermore, a method of adding a material for improving the safety of a battery to an electrode active material or coating the electrode active material with such a safety improving material does not solve the fundamental problem in that the performance of the battery is deteriorated during the normal operation of the battery. Also, the material is directly added to components related to the operation of the battery, and therefore, the amount of the material added is restricted.